The discovery of graphene, the first single-atom thick material, in 2004 [1]}, [2]}, has accelerated research into atomically thin two-dimensional (2D) materials. The capacity to control isolated single atomic layers and reassemble them layer-by-layer in a precise sequence to build heterostructures opens up endless possibilities for applications [3]}, [4]}, [5]}. Semiconducting dichalcogenides are attractive compounds in this regard since they can be readily exfoliated and have a suitably narrow gap both in bulk and as a single layer.
Monolayer molybdenum disulfide (MoS2) has recently attracted attention in this type of 2D dichalcogenides systems for combining electron mobility comparable to graphene devices with a limited energy gap [6]}. Monolayer MoS2 possesses a direct gap, which is an indirect gap semiconductor [3]}, [8]}, making it particularly intriguing for optoelectronics. Another intriguing aspect is that the electrical properties appear to be very sensitive to external pressure [9]}, strain [10]}, [11]}, and temperature [12]}, which influence the gap and, under certain conditions, can also produce an insulator/metal transition. Furthermore, the lack of lattice inversion symmetry combined with spin-orbit coupling (SOC) leads to coupled spin and valley physics in monolayers of MoS2 and other group VI dichalcogenides [13]}, allowing control of spin and valley in these materials [14]}. Because of their unique band structure, monolayers of MoS2 have been proposed for a range of nanoelectronics applications [4]}, including valleytronics, spintronics, optoelectronics, and room temperature transistor devices [6]}.
